Members of the United Steel Workers union picket the Tesoro refinery in Carson, California February 2, 2015. REUTERS/Bob Riha, Jr.

NEW YORK (Reuters) - Workers represented by the Building Trades Unions will return to Marathon Petroleum Corp’s (MPC.N) refinery in Catlettsburg, Kentucky on Monday, even as a strike by the United Steelworkers drags into a second week, documents obtained by Reuters show.

The USW began a strike Feb. 1 at nine refineries and chemical plants, including Marathon’s 240,000 barrel-a-day Kentucky plant.

Buildings Trades Unions’ workers will not be replacing striking workers’ running the plant, according to the letter.

While the Building Trades Unions say they continue to support the USW’s cause, their return work comes at a sensitive time for the steelworkers as their strike enters a second week. The backing of the building unions is considered crucial for a successful strike nationally, a person familiar with refining said.

For the first week of the strike, Marathon advised workers represented by the Building Trades Unions, not to return to work. These unions represent workers who are employed by maintenance contractors at the plant, while USW workers generally operate the units. The latest letter tells members to return to work Monday, but not to replace USW workers who are still on strike.

“As a labor organization, we fully respect the USW workforce,” the business manager of the building trades said in a letter to local unions. “However to avoid any possible detrimental legal proceedings, it is advised that your local union direct your members to report to work.”

Talks between the USW and Shell, which is negotiating on behalf of the oil companies at a national level, have been recessed. The USW has rejected six contract offers so far.